As the name suggests, material handling machines are used to move material from one place to another. Both the machines themselves and the material handled by them can take any of a wide variety of forms. For example, material handled by such machines may include excavated soil, rock, steel coils and plate, ladles of molten metal, lumber and logs. Machines for handling such material include excavators, loaders, shovels and cranes. The latter includes overhead travelling cranes, which run on elevated rails, and gantry-type cranes, "stiff-legged" structures which run on rails mounted at ground level.
While the aforementioned types of cranes come in a variety of configurations, they share some common features. One is that they both ride on rigid flanged wheels rolling atop parallel, spaced railroad-like rails. Another is that they are equipped with a traversely-movable trolley on which is mounted a load-lifting hoist apparatus. Such cranes are capable of picking up a load from a location, raising it to an elevation and transporting the load to another location. Yet another common feature is that they are capable of moving a load in any of three axes of motion, namely, travel (along the rails), trolley traverse (perpendicular to the rails) and hoist/lower.
Gantry cranes have a generally horizontal frame arrangement supported above the ground on side supports so as to span the area beneath the frame. Gantry cranes are often referred to as portal cranes since when viewed parallel to the rails, they appear gate-like with a large opening defined by the frame structure and the side supports. A type of portal crane is shown in U.S. Pat. No. 5,022,542 (Beier).
The exemplary crane shown in the Beier patent has downwardly-extending legs, each terminating in a wheel assembly. While a wheel assembly on either side of the crane may simply be a non-driven "idler" assembly, each side of the crane has at least one wheel assembly which includes a drive motor and gearing. The operator controls movements of the crane from a control cab well above the ground.
Portal cranes are often used out-of-doors and are subjected to certain environment-related and use-related forces. An example of an environment-related force is wind loading. Wind in a direction parallel to the rails imposes forces on the crane which tend to tip the crane. In recognition of this and other factors, the support legs of portal cranes generally define a triangle with spaced-apart lower "feet" for stability. Portal cranes are often made in a lattice arrangement for, among other reasons, reducing the frontal area subjected to wind-imposed forces.
An example of a use-related force involves the control of the drive motor(s) at either side of the crane. It presents a very difficult control problem to make all motors run and brake at precisely the same rotational speed and at the same time. As a result, the crane can become slightly twisted or skewed by the motor(s) on one side driving that side slightly faster than the motor(s) on the other side.
And even if all of the motors on both sides of the crane are operated in precise synchronism, other forces are imposed on the crane by merely accelerating and decelerating it as a load is moved from one location to another. As a rough analogy, the head of an automobile driver or passenger tends to move rearward if the auto is accelerated sharply or forward if it is braked abruptly. The suspended horizontal girder structure of a portal crane experiences the same kind of forces during crane acceleration and deceleration.
Yet another example of a use-related force involves the load itself. Such load is suspended from cables which are taut but not rigid. When the crane is accelerated, the load lags slightly and tends to twist the horizontal, suspended frame about its long axis. Deceleration produces the same type of twist but in the opposite direction.
The portal crane shown in the Beier patent has a tubular top chord welded to a supporting plate. In turn, the plate is welded and bolted to a horizontal girder support beam. The upper ends of diagonal "laces" or tube members are welded to the top chord and their spaced lower ends are attached through a reinforcing, pin-attachment plate to the lower ends of support arms or links.
Forces transferring between the horizontal girder beam and the downwardly-extending legs are imposed on the welds attaching the tubular top chord and the diagonal laces to one another. Such forces are also imposed on the weld securing the top chord to its support plate and upon the bolts securing the support plate to the girder beam. And such force transfer is not only inevitable, it occurs substantially continuously when the crane is being used and, to some degree, if the crane is subjected to high wind forces, whether or not in use.
With the above-noted earlier configuration, it has been found unexpectedly that failures may occur in any one, some or all of three locations, namely, on the top chord/laces welds, on the top chord/plate weld and/or on the plate/support beam weld and bolts. A logical approach to preventing such failures would be to "beef up" and reinforce the welds, bolts and the like. However, the invention substantially eliminates these failures in a very unusual way using a unique force transfer link and a "floating" girder arrangement.